Servo controllers in general are demanded to follow a command quickly and precisely and, in order to accomplish this, use two-degrees-of-freedom control which combines feed-forward control and feedback control. However, there are limitations in torque and other forms of control input that an actuator such as a motor can generate. In addition, there are limitations in the rate of change of control input, which is related closely to a voltage generated in a servo controller and, when increased, causes the voltage to rise to the point of voltage saturation and also magnifies shock to the mechanical system. Desired control is therefore to achieve quick, precise following to a reference while staying within the limitations of control input in terms of saturation and rate of change.
On the other hand, when characteristics in following to a reference are constant, the limitations of control input in terms of saturation and rate of change pose a problem in varying degrees depending on how large/small and how rapid a change to the reference is. In particular, controlling a vibratory plant which has mechanical resonance so that vibrations excited in following a command are actively cancelled has a problem of increasing control input and the rate of change of control input to excess, thereby facilitating saturation.
A conventional servo controller described in Patent Literature 1 has the configuration of a two-degrees-of-freedom control system and includes, in a feed-forward control unit, a mathematical model approximating a plant, FF position control, and FF speed/stabilization control. The FF position control uses position proportional control of a fixed gain or a predictive controller that utilizes command prefetching. The FF speed/stabilization control is constituted of speed PI control and state feedback of a mathematical model whose gains are fixed.
A conventional servo controller described in Patent Literature 2 includes switches to improve its performance by employing feedback control that uses a predictive controller as a feedback controller, or two-degrees-of-freedom control, when the switches are turned on, and switching to feedback control that involves position proportional control alone when the switching switches are turned off. In this conventional example, the switching switches are turned on when a plant requires precise following characteristics, and are turned off when the targeted command increment value is changed. The conventional example also involves linear interpolation of control input before and after switching which is performed by a filter function part in order to give continuous values to control input before and after switching.